For 3.3V devices, the levels are typically a factor of the supply voltage.
For ATMega328s:
With Vcc = 3.3V:
Low Input must be <= 0.3 x Vcc, = 0.99V
High input must be >= 0.6 x Vcc = 1.98V
1V to 1.97V: input is not defined, may be seen as high or low.
With Vcc = 3V:
Low output will not higher than 0.6V with 10mA load
High output wil not be less than 2.3V with 10mA load
Numbers are from Section 29 of the '328 datasheet.